This invention relates to an apparatus and method for mixing liquid or drilling mud with solid or liquid materials and more particularly to a multi-stage centrifugal mud mixing device utilizing high rotational velocity for obtaining a homogeneous mixture of slurry and added materials.
In drilling for any hydrocarbon products it is necessary to control the hydrostatic head of the drilling mud at the bottom of the drill hole. The drilling mud is used for purposes of preventing geopressured hydrocarbon materials from coming to the surface. At the bottom of the hole surrounding the geopressured hydrocarbon materials are natural gases that are also under pressure, this pressure may be defined as formation pressure. The hydrostatic head of the drilling mud must be greater than this formation pressure to prevent the drilling mud from being blown out of the hole.
A second problem encountered in drilling for hydrocarbon materials is in bringing cuttings from the drill to the surface of the hole, that is, loose rock and debris cut by the drill bit from the bottom of the hole. A mud slurry is also injected into the hole for purposes of floating or carrying up these cuttings from the bottom of the hole.
For each of the above uses of mud slurry in the drilling operation for hydrocarbon products the density of the mud slurry as well as its viscosity is of great importance. For example, the deeper the drill hole the greater the formation pressure of the hydrocarbons found at the bottom of the hole and therefore the greater the mud slurry density required to maintain the proper hydrostatic head at the bottom of the hole. Overbalancing of the formation pressure by the hydrostatic head at the bottom of the drill hole prevents blow-out from the hole of natural gases and other hydrocarbon products as stated above.
Further, hydrocarbon drilling operations require the use of mud having a viscosity such that when injected into a drill hole will allow cuttings to be carried to the surface. This type of viscous mud slurry is obtained by mixing clay, or bentonite with water. In order to obtain the proper mud density for controlling the hydrostatic head at the bottom of the drill hole a mud slurry mixture is further concentrated with high density materials, such as barium sulfate, i.e. barite.
The prior art teaches several methods and devices for controlling the density and viscosity of mud slurry used in hydrocarbon drilling operations. One type of device deals with addition systems, which may be defined as a device connected to a continual flow system for purposes of injecting a second material into the continuous stream. The only actual mixing performed in such an addition system is any mixing that can be obtained from the movement of the flow material in its confined passageway. The addition system may be merely a second passageway connection for a liquid addition, or may be a funnel holding solid materials connected by a sleeve into the continual flow passageway.
As stated above an addition device will not actually perform a mixing operation, however, also taught in the prior art is a device having a solids hopper or funnel connected to a mixing chamber having an inlet passageway for providing a liquid or slurry to be mixed with solid materials. Mixing in this type of apparatus is enhanced by the use of a jet nozzle passageway carrying the mud slurry or liquid material into the chamber. The mud slurry or liquid is jet sprayed horizontally into the chamber as the solid materials are axially dispersed into the mixing chamber. Further mixing is accomplished in this device by attaching a venturi to the discharge port downstream from the jet mixer. The venturi provides reduction and enlargement of the discharge port which causes velocity change in the slurry thus enhancing turbulence before discharge and recovers part of spent energy. A distinct disadvantage of this venturi based mud mixing device is that it continually plugs with the solid materials which are axially fed into the mixing chamber and surround the jet spray. Since the vacuum created in the mixing chamber is not sufficient to assist in discharging the solids through the slurry, and the jet spray being only unidirectional cannot pick up all solids surrounding the inlet passageway and solid material, build-up results which requires manual cleaning before further use of the device. A further drawback of this type of system is in the capacity which is dependent upon the amount of port size reduction in the venturi. Although the capacity may be enhanced by a decrease in the port size reduction of the venturi, this expansion will detract from the mixing action caused after the reduction.
The prior art further discloses a mixing device utilizing two inlet ports to an annular mixing chamber having an axial extension passageway connected thereto. By applying a fluid into one inlet of the annular chamber tangentially, a high rotational velocity is obtained within the mixing chamber causing a vortex or air core to be formed in the axially extended passageway. A second fluid is interjected by a second inlet port into the mixing chamber axially and mixed with the first fluid by the rotational forces of the first fluid in the mixing chamber. As the mixture moves in the axial extension of the mixing chamber it continues to rotate in the same direction as the fluid in the annular housing. However, as the fluid is dispelled from the axial extension of the mixing chamber into a second chamber, before being discharged through a discharge port, the vortex is destroyed. This causes further turbulence of the fluids for mixing purposes and begins rotation in an opposite direction to that of the fluids in the mixing chamber. Such a device is disclosed in U.S. Pat. No. 2,957,495 by Ashbrook. This Ashbrook device makes no provision for injecting solid materials into the annular mixing chamber. Primarily used for mixing fluid into fluid or gas into fluid, any attempt to mix solid into a fluid would cause plugging in the device axial extension passageway of the annular chamber and render the device inoperative. Furthermore, high density materials, such as barite for example, not being flowable materials would render such a system as that found in Ashbrook inoperable since a nonflowable material would not be able to pass through the turn in the inlet passageway in the manner disclosed in Ashbrook without proper pumping of the solid.